Does SPD MCB size matter?

[oscar21]

Does this matter? I've fitted a Fusebox consumer unit and I didn't have enough 32A MCB's but too many 6A's. The SPD is fed from a 32A one, what's the deal with swapping them around. They are voltage operated not current operated so should be fine right?

 

[James]

check the instructions! they will surely tell you what breakers can be used with the spd?

 

[Markyd]

It has come from the manufacturers with a 32 amp mcb for the spd....Why do you want to mess with that, its made like that for a reason.....buy another mcb, and really you should be fitting rcbo boards

 

[SJD]

A couple of days ago, I retrofitted a Contactum SPD kit (into a consumer unit I'd installed a few years previously) - the MCB supplied in the kit was a 63A C-type. I suspect a low value MCB might trip when instead the energy should be going into the SPD.

 

[oscar21]

It has come from the manufacturers with a 32 amp mcb for the spd....Why do you want to mess with that, its made like that for a reason.....buy another mcb, and really you should be fitting rcbo boards

RCBO boards are just a vanity thing, nothing wrong with split load boards and I'll happily fit one if the customer wants one. In fact some of the council specs we get specify a split load board, usually a Wylex one and they even provide the model number for one
(this is most probably because they haven't got a jar of glue what they are doing and just copy and paste googled stuff into the specs to make themselves look clever and keep their job) but that's bye the bye.

As for the SPD what's so special about a 32A one, Have you seen what lightning does when it strikes, millions and millions of volts destroy trees, buildings, vaporise most things they strike, it jumps thousands of feet with an arc brighter than the sun, the most 230V mains has ever done to me is put a little hole in my snips. Does it really make that much difference if a 6A or 32A MCB is on the receiving end of a bolt of lightening?

 

[DPG]

RCBO boards are just a vanity thing, nothing wrong with split load boards and I'll happily fit one if the customer wants one. In fact some of the council specs we get specify a split load board, usually a Wylex one and they even provide the model number for one
(this is most probably because they haven't got a jar of glue what they are doing and just copy and paste googled stuff into the specs to make themselves look clever and keep their job) but that's bye the bye.

As for the SPD what's so special about a 32A one, Have you seen what lightning does when it strikes, millions and millions of volts destroy trees, buildings, vaporise most things they strike, it jumps thousands of feet with an arc brighter than the sun, the most 230V mains has ever done to me is put a little hole in my snips. Does it really make that much difference if a 6A or 32A MCB is on the receiving end of a bolt of lightening?

RCBO boards are definitely not just a vanity thing. For the slight extra cost why would you not fit RCBOs? Especially ones that switch the neutral as well.

And the lightning thing - you do realise an SPD does not protect against direct strikes right? That's the job of a lightning protection system - different kettle of fish. An SPD protects against voltage spikes on supply cables.

 

[littlespark]

A 6A would possibly trip well before the SPD had a chance to operate, rendering it useless. For the cost of another MCB…

As for RCBO vs dual rcd… it’s wrong to say he should fit an all RCBO board. There’s nothing in the regs preventing using a dual rcd….. and it might be all the customer can afford.

 

[James]

I say again, read the instructions for the SPD.

 

[oscar21]

RCBO boards are definitely not just a vanity thing. For the slight extra cost why would you not fit RCBOs? Especially ones that switch the neutral as well.

And the lightning thing - you do realise an SPD does not protect against direct strikes right? That's the job of a lightning protection system - different kettle of fish. An SPD protects against voltage spikes on supply cables.

Why not fit RCBO's, well its a unit that goes into a cupboard and in all likelihood will never be touched or seen again. As stated elsewhere there is nothing wrong with fitting a dual RCD board and as long as the customer is safe from electrocution nothing else really matters.

How often in reality do you think an RCD trips? my own dual rcd board thats been in about 13 years now has never tripped except when I've been fiddling about with things, I would imagine in the grand scheme of things with all the RCD's in existence most of them never trip in their lifetime either.

Also even if an RCBO trips it still needs fixing, you cant just leave it off and say well at least we've still got the other 9 circuits to use. So you still need to call out an electrician to fix it as soon as and if the power is off most will turn up the same day anyway if you sound suitably distressed on the phone. Personally I think dual RCD is adequate enough in a domestic setting.

As for SPD, what I am trying to say is what makes a 32A breaker suitable when a 6A one isn't, why not a 40A or 63A one are all spikes less than 32A then? I also don't buy into all this SPD nonsense in the first place anyway. My TV broke after 3 years the other week, I just put it down to crap build quality, never once did I think its due to the lack of SPD in my CU, its just like every other piece of plastic junk that has a built in life span,. If SPD's prolonged the life of disposable tat they would never be allowed.

 

[DPG]

Why not fit RCBO's, well its a unit that goes into a cupboard and in all likelihood will never be touched or seen again. As stated elsewhere there is nothing wrong with fitting a dual RCD board and as long as the customer is safe from electrocution nothing else really matters.

How often in reality do you think an RCD trips? my own dual rcd board thats been in about 13 years now has never tripped except when I've been fiddling about with things, I would imagine in the grand scheme of things with all the RCD's in existence most of them never trip in their lifetime either.

Also even if an RCBO trips it still needs fixing, you cant just leave it off and say well at least we've still got the other 9 circuits to use. So you still need to call out an electrician to fix it as soon as and if the power is off most will turn up the same day anyway if you sound suitably distressed on the phone. Personally I think dual RCD is adequate enough in a domestic setting.

As for SPD, what I am trying to say is what makes a 32A breaker suitable when a 6A one isn't, why not a 40A or 63A one are all spikes less than 32A then? I also don't buy into all this SPD nonsense in the first place anyway. My TV broke after 3 years the other week, I just put it down to crap build quality, never once did I think its due to the lack of SPD in my CU, its just like every other piece of plastic junk that has a built in life span,. If SPD's prolonged the life of disposable tat they would never be allowed.

Yeah I get the point that a dual RCD board is often sufficient. But RCBOs have got to be the preferred option, especially with the issue of cumulative leakage.

The thing is you will never know if your equipment packed up due to a general fault, or due to a voltage spike on the supply. I recall a few years ago when an equipment manufacturer tried to save costs by missing out certain protection components in a modem circuit of a product. Over the next few months/years it became apparent that more devices were being returned as faulty, particularly from certain areas of the globe. The areas which were likely to suffer from storms.

 

[oscar21]

I do get the point about SPD's and they obviously reduce spike in the supply but they will never be attributed to saving damaged goods. Our assesor was trying to "sell" them to us a few years ago saying if we didn't fit them and then 3 months later the customers TV broke the we would be in hot water and liable for a new TV.

Can you imagine the scenario, apart from anything else it will be under guarantee but lets say its just out, the customer can either buy another TV for a few hundred pounds or take me to the small claims court, once in court the customer says "the lack of a SPD damaged my TV" I reply "prove it" that's all there is to it really. I suppose the customer could then go away and pay for expensive engineers reports and stuff but how would they even prove the spike didn't come from within the TV itself. Its just not going to happen and disposable goods and going to continue to break at an alarming rate just out of warranty.

 

[DPG]

I do get the point about SPD's and they obviously reduce spike in the supply but they will never be attributed to saving damaged goods. Our assesor was trying to "sell" them to us a few years ago saying if we didn't fit them and then 3 months later the customers TV broke the we would be in hot water and liable for a new TV.

Can you imagine the scenario, apart from anything else it will be under guarantee but lets say its just out, the customer can either buy another TV for a few hundred pounds or take me to the small claims court, once in court the customer says "the lack of a SPD damaged my TV" I reply "prove it" that's all there is to it really. I suppose the customer could then go away and pay for expensive engineers reports and stuff but how would they even prove the spike didn't come from within the TV itself. Its just not going to happen and disposable goods and going to continue to break at an alarming rate just out of warranty.

Or you just advise the customer that an SPD is a very good idea for a very small amount of money. Then fit one, and everyone is happy.

For around £60 you can get a metal Fusebox 10 way consumer unit with SPD. Type A RCBOs are about a tenner.

 

[oscar21]

Or you just advise the customer that an SPD is a very good idea for a very small amount of money. Then fit one, and everyone is happy.

For around £60 you can get a metal Fusebox 10 way consumer unit with SPD. Type A RCBOs are about a tenner.

We fit them all the while now, its not worth it not to nowadays as they are so cheap, this was a few years ago the asseser was trying to get us to fit them at a big cost to ourselves when they weren't even mentioned in the regs, he probably had shares in them or something. We use the Fusebox ones a lot but this council work we do usually spec Wylex which are so expensive for the RCBO's. so when it says dual RCD on the spec one of those go in, even though the spec is mostly gobledygook.

 

[DPG]

We fit them all the while now, its not worth it not to nowadays as they are so cheap, this was a few years ago the asseser was trying to get us to fit them at a big cost to ourselves when they weren't even mentioned in the regs, he probably had shares in them or something. We use the Fusebox ones a lot but this council work we do usually spec Wylex which are so expensive for the RCBO's. so when it says dual RCD on the spec one of those go in, even though the spec is mostly gobledygook.

Yeah does sound like they just haven't kept up to date.

So the dual RCD ones you fit - do they have SPDs?

 

[oscar21]

Yeah does sound like they just haven't kept up to date.

So the dual RCD ones you fit - do they have SPDs?

Yes, Like I say, if its private work we fit the Fusebox ones and if its council we fit also fit the Wylex ones with SPD's, Edwardes have a good deal on the wylex ones at the moment.

Off track slightly but the new Wylex stuff is pretty crap, you can't remove the din rail so have to take everything to bits just to get the RCD's out, they are exactly the same as the new Crabtree stuff as well so it all comes out of the same factory. And those metal blanks that slide about all ove the show when you are trying to get the lid on, woeful, although on the last board I did it came with different blanks that could clip in from the front.

 

[nicebutdim]

Yes, Like I say, if its private work we fit the Fusebox ones and if its council we fit also fit the Wylex ones with SPD's, Edwardes have a good deal on the wylex ones at the moment.

Off track slightly but the new Wylex stuff is pretty crap, you can't remove the din rail so have to take everything to bits just to get the RCD's out, they are exactly the same as the new Crabtree stuff as well so it all comes out of the same factory. And those metal blanks that slide about all ove the show when you are trying to get the lid on, woeful, although on the last board I did it came with different blanks that could clip in from the front.

Metal blanks can be a pain, but I understand they're used not only because it's a 'non combustible' material, but also because they require a tool to remove, whereas traditional blanks can be pulled out easily.

Wylex make blank modules that clip on the din rail and these also fit Crabtree starbreaker boards.

 

[loz2754]

Wylex make blank modules that clip on the din rail

As do Fusebox and Hager. I only use this type now.

 

[nicebutdim]

As do Fusebox and Hager. I only use this type now.

Some can be a pain to remove, but for very little extra money they look like a finished job.

 

[pc1966]

Does it really make that much difference if a 6A or 32A MCB is on the receiving end of a bolt of lightening?

Yes, but not for the obvious reasons.

As already mentioned most SPD are not rated to deal with a direct strike. The 'type 2' that most folks fit are intended for the 8us/20us waveform typical of induced lighting surges and similar when a strike hit something else and the power cables pick up the radiated EM disturbance.

They 'type 1' (or more commonly type 1+2) are rated for a portion of a direct strike, so if your building has a Lightning Protection System fitted then the assumption is a portion of the direct hit goes via the LPS to Earth, but some of it goes via the power cables and then the SPD has to divert it. This may have similar peak current ratings in the 10-50kA region but now the waveform is a 10us/350us shape (i.e. falr longer tail of decaying current) and as such it has about 17 times higher energy for a given current peak. That is why those SPD are often double-width and 5 times the price!

Back to the MCB, the problem here is inductance as at high frequencies (and the bulk of lightning power is around 100kHz, not 50Hz) that dominates circuit impedance over conduction resistance in most situation. That is why the regs have the 0.5m / 1m max cable length when fitting an SPD otherwise the series impedance of the cable loop increases how much voltage passes by to the supposedly protected installation.

When comparing a 6A MCB and a 32A MCB the "instant" magnetic trip point (instant in human terms, which is still several milliseconds, not microseconds) the trip point is just over 5 times higher, so your trip coil needs (for assumed same mechanical arrangement) 5 times the number of turns so the fault current applies the same force to initiate tripping. But inductance is proportional to turns squared, so your impedance to high frequencies is over 25 times higher!

 

[Flanders]

A 6A would possibly trip well before the SPD had a chance to operate, rendering it useless. For the cost of another MCB…

As for RCBO vs dual rcd… it’s wrong to say he should fit an all RCBO board. There’s nothing in the regs preventing using a dual rcd….. and it might be all the customer can afford.

See Reg 531.3.2 unwanted tripping (ii) rcbos should be considered. Also see section 314